1. Field of the Invention
The present invention relates generally to a method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride using a gas diffusion electrode as the cathode.
2. Description of Related Art
The electrolysis of aqueous solutions of hydrogen chloride (hydrochloric acid) and aqueous alkali metal chloride solutions can be conducted electrolytically using gas diffusion electrodes as oxygen-consuming cathodes. In such a case, oxygen, air or oxygen-enriched air are fed in excess into the electrolysis cell. By using oxygen-consuming cathodes, the electrolysis voltage is reduced by approx. 30% compared with conventional hydrochloric acid or chloralkali electrolyses.
A method for the electrolysis of hydrochloric acid is known, for example, from U.S. Pat. No. 5,770,035. An anode space with a suitable anode, including, for example, a substrate made of a titanium-palladium alloy, which is coated with a ruthenium, iridium and titanium mixed oxide, is filled with hydrochloric acid. The chlorine formed at the anode escapes from the anode space and is subjected to further processing. The anode space is separated from the cathode space by a commercially available cation exchange membrane. On the cathode side, there is a gas diffusion electrode (oxygen-consuming cathode) adjoining the cation exchange membrane. The oxygen-consuming cathode in turn adjoins a current distributor. The oxygen supplied to the cathode space is reacted on the oxygen-consuming cathode.
EP-A 1 067 215 discloses a method for the electrolysis of an aqueous alkali metal chloride solution using an oxygen-consuming cathode. The electrolysis cell includes an anode half-element and a cathode half-element, which are separated from each other by a cation exchange membrane. The cathode half-element includes an electrolyte space and a gas space. The electrolyte space is separated from the gas space by an oxygen-consuming cathode. The electrolyte space is filled with an alkali metal hydroxide solution. The gas space is supplied with oxygen. The anode space is filled with a solution containing alkali metal chloride.
The formation of hydrogen on oxygen-consuming cathodes should generally be prevented. Inter alia because of competition reactions, however, traces of hydrogen may even be formed if the oxygen supply is sufficient. The hydrogen is drawn off from the cathode space together with the excess oxygen. According to previous methods, the oxygen is released into the waste air after off-gas purification, since recycling risks that hydrogen concentration will rise to above its explosion limit of 4 vol. %.